with
blankets
and Varn Chemicals
TM
Guidelines
Guidelines to Better Printing
Introduction
To our fellow pressroom professionals and
customers: this offset printing blanket and
chemical overview is provided to give the
press operator a basic understanding of
printing blankets and chemicals and enhance
their ability to produce trouble-free, high
quality print jobs.
The information is excerpted from materials
used by Day International’s Technical Support
and Pressroom Training Team.
If you have a specific problem or question that
is not answered in the following materials,
you may contact Day International’s Technical
Support by calling our Customer Service
Department at 800-877-8187, ext. 0.
Table of Contents
1
Offset Printing Blanket Overview ...........................1
2
Blankets On The Press.......................................................9
3
Care and Maintenance
of Offset Blankets .................................................................15
4
Measures To Ensure Optimal
Blanket Performance .........................................................16
5
Varn Product Overview.....................................................18
6
Roller and Blanket Maintenance.............................19
7
VOC and Vapor Pressure ...............................................22
8
Fountain Solution Chemistry ..................................24
1
Offset Printing Blanket
Overview
Purpose of Blankets
The offset printing blanket is the most important element in the offset
printing process. It’s the final point of contact between the press and the
printed substrate. The purpose of the blanket is to transfer the inked
image from the plate to the substrate without any distortion of the image
in its original form; so selecting the proper blanket for your particular
application is a critical factor in obtaining optimum print quality. Inferior
blankets or improper blanket techniques can cost you money in downtime, waste and quality. They eventually can cost you valuable customers.
Blanket Types
There are two basic types of offset printing blankets — compressible and
conventional. Compressible blankets are the most popular blankets today
because of better smash resistance that results in longer life and wider
packing latitude. This allows the blanket to be slightly over or under
packed and still produce acceptable print quality. Compressible blankets
contain a cellular sponge-type layer that functions in a manner similar to
a shock absorber.
Conventional, or non-compressible blankets continue to be used today in
applications where high print pressures are required. Conventional
blankets are sometimes referred to as “hard” blankets. They require
precise packing methods and offer very little packing latitude.
1
COMPRESSIBLE
CONVENTIONAL
Printing Surface
Fabric
Compressible Layer
Fabric
Blanket Construction
Offset printing blankets are typically constructed of two basic components
—a carcass and a surface layer.
The Carcass
The carcass, or body of the printing blanket, is usually made up of
several layers of fabric (ply). The layers are laminated together with
adhesive rubber cement and, in the case of a compressible blanket, a
compressible layer.
The fabric layers, which can range from as few as two-ply to as many as
five-ply depending on thickness requirements, are specially designed for
offset printing blankets and contain natural and synthetic fibers.
Each layer or ply of fabric in the blanket may have its own unique physical characteristic. The adhesive cements are designed to have high bond
strength along with resistance to a wide variety of pressroom chemicals.
The compressible layer in dayGraphica brand blankets is composed of
tiny air bubbles created by the use of micro-spheres surrounded by a
resilient synthetic rubber compound. This ‘closed-cell’ type of compressibility offers a very even distribution of air cells throughout the compressible layer which provides consistent print pressures as well as excellent
recovery in the event of a blanket smash. Additional benefits provided by
a compressible blanket include:
• Reduced plate wear
• Reduced mechanical problems (gear vibrations, etc.)
• Increased packing latitudes (for varying stock thickness)
The Surface Layer
The rubber compound and finish of the blanket surface plays an important role in determining the blanket’s print quality characteristics.
Blended, synthetic rubber compounds are used because they can be formulated to be compatible with various inks, fountain solutions and washes.
Synthetic elastomers have replaced the natural rubbers previously utilized
in the surface or “face” of the blanket. The type or blend of elastomers used
in a blanket depends on the intended application. In addition to the type of
synthetic rubber compound, the blanket’s texture or surface finish also
plays an important role in the performance of the blanket.
2
Offset Printing Blanket Overview — continued
There are three different types of surface finishes:
1.
dayGraphica
8500
2.
3.
dayGraphica
3000
Cast Surface—a finish produced by using a special type of paper
inter-leafed against the rubber face during the curing process. It’s a
common blanket surface finish characterized by a smooth, shiny
appearance.
Ground Surface—produced by a mechanical grinding process
after the blanket has been cured. It requires an additional manufacturing step but allows the manufacturer to achieve the close thickness (gauge) tolerances and offers various degrees of roughness
profiles on the blanket surface for different printing applications.
Textured Surface—a surface finish produced through a chemical
process. This process is an exclusive Day International patented
process, which produces a cast surface blanket without talc and
with the option of various degrees of roughness. This surface
texture, in combination with the face rubber, is widely accepted as
having the best release characteristics of all blankets.
The three surface types offer different advantages to the printer. For best
printing results, it’s important for the printer to determine which surface
finish is best suited for the particular application (the blanket manufacturer should be able to provide guidance in this issue).
dayGraphica 9500
Important Physical Characteristics
of Blankets
• Compressibility—rubber by itself displaces when force is applied, so
air has to be trapped inside it to allow it to compress.
Compressibility is the blanket’s ability to reduce in volume under
pressure and then rebound to its original state. Compressibility also
allows the blanket to exert approximately equal amounts of point
pressure over a range of packing levels.
• Tensile Strength—an important characteristic of the fabric in a
printing blanket is its tensile strength. Since most printing blankets are
tensioned on the blanket cylinder, they must be able to resist breaking
at high levels of force or tension.
Tensile strength is a measure of the break strength of the blanket. The
design of the fabrics and the number of fabric plys in the blanket affect
its tensile strength.
• Stretch—another important blanket characteristic is the percentage of
elongation under a given load. It’s vital for a blanket to have a small
amount of stretch or elongation so that it will conform to the blanket
3
cylinder. Precise control of blanket stretch characteristics is critical to
ensure consistent installation of the blanket on the press.
It’s recommended that torque wrenches are calibrated and pre-set to
blanket and press specifications when tension is applied to the blanket.
• Surface Hardness—it’s necessary to differentiate between surface
hardness and blanket durometer. Because of the proximity of fabrics
near the face of the blanket, blanket durometer (hardness measurements) of the blanket can be misleading when measured with a handheld shore durometer.
It’s likely if you measure two blankets, each with a very different
surface hardness, your readings will be very similar because of the
fabric influence on the reading of the overall blanket. A more accurate
measurement of the surface hardness is obtained by using a microhardness tester.
• Surface Roughness—the surface profile of a printing blanket has a
direct effect on its print quality characteristics. A rough surface blanket
offers excellent mechanical release properties but will transfer a
somewhat jagged-edged dot. A smooth surface will produce better dot
structure but with less mechanical release. The surface roughness of a
blanket is typically reported as an average value and measured with a
Profilometer.
• Thickness—an offset printing blanket should be uniform in thickness.
Typically, cut blankets less than 42" wide should vary no more than
.001". When measuring the thickness of a blanket, be sure to use a
constant force micrometer or a gauge designed and calibrated to
measure blankets or soft materials. Measure the blanket, rubber side
up, in at least three locations on each end and side of the blanket.
• Squareness—due to the directional, low-stretch characteristics in the
“around the cylinder” direction of the blanket, it’s important the blanket
is cut square. Measure the blanket from opposite corners to check for
squareness. Variation should be no more than 1/16". Blankets cut out of
square will not tension properly on the press and may possibly cause
print related problems and web control problems such as wrinkling.
• Solvent Resistance—blankets are exposed to a wide variety of
chemicals in the pressroom. They must be able to resist swelling and
delamination caused by inks, fountain solutions and washes. (Day
International offers controlled laboratory chemical compatibility testing
for all of your pressroom chemicals. Refer to Chapter 5 — Varn Product
Overview.)
4
Offset Printing Blanket Overview — continued
Blanket Characteristics
Not all printing blankets are created equal.
Although printing blankets may appear similar on the surface, there are a
number of factors to consider when selecting the proper blanket for your
press and printing application.
Printing blankets have many physical properties that vary depending
upon the manufacturer and the blanket style. Characteristics such as:
• print quality
• ease of cleaning
• release
• ease of installation
• durability
• ease of removal
All these properties should be considered when trying to decide on the
best blanket to suit your printing needs. The best assurance of choosing a
quality blanket is to buy from a reputable manufacturer that can consistently supply you fresh products with uniform quality in every delivery.
Some additional features of a high quality offset printing blanket should
include:
• Identification marking—on the fabric side of the blanket, there
should be a way to identify the manufacturer and the style of the
blanket. The markings should include:
• name of manufacturer
• size (around x across)
• roll lot number
• blanket gauge (thickness)
• blanket style
• Color stripe woven into the fabric of the blanket—these color
stripes identify the manufacturer and indicate the direction in which the
blanket should be mounted on the cylinder. dayGraphica blankets
display a gold color fabric stripe on every blanket. The fabric stripe
should go around the cylinder, not across.
• Bars or end treatments—generally your blanket supplier can provide
blankets with bars already attached. These bars should be neatly and
securely attached to each end of the blanket without any excess epoxy
or residue on the surface or fabric.
If your blankets require holes or have to be punched, these holes
should also be neat and aligned properly on the ends of the blanket.
For all blankets, squareness is important. Be sure to check all blankets
to make sure they are cut properly.
• Rubber surface color—the pigment used to produce different colors
of printing blankets usually does not affect the performance of the
blanket. However, extreme color variation within an individual blanket
or blankets of the same style but from a different roll lot number may
indicate aged or inferior quality blankets. Slight color variations are
5
common with all rubber products, but if the variation is significant, it
is recommended you check with your supplier before using the blanket.
Although the color of the rubber, for the most part, is irrelevant to the
performance of the blanket, certain surface colors can aid the printer in
reverse image inspection and troubleshooting.
• Edge Sealants—at the request of the customer, a water and solvent
resistant coating can be applied to the edges of the blanket. This will
help keep moisture from penetrating the carcass of the blanket and
prevent swelling and delamination.
The sealant should be applied only to the edges of the blanket with no
excess on the surface or fabric side.
Evaluating Blanket Performance
It’s difficult to predict performance results of a printing blanket for each
pressroom application because of the many variables that contribute to
the performance of the blanket; variables such as:
• press mode
• fountain solution
• type of ink
•· press operating procedures
• paper stock
• press speeds
All these variables play a part in the performance of a printing blanket.
So, it’s important that you work to determine which blanket best suits
your individual needs.
Here are some of the items that should be considered when evaluating
the performance characteristics of an offset printing blanket.
• Print Quality—the print quality produced by a printing blanket is
probably the most critical characteristic to consider. Blanket test comparisons should be obtained from information gathered on new blankets
currently being used and on new blankets that you wish to test.
If test information indicates improved results from the test blankets,
continue production and evaluate longevity and endurance between the
styles of blankets currently being used and the new test style blankets
(print quality of a blanket sometimes changes after it has been on press
for awhile).
There are several testing methods available today. For an accurate
comparison of the print quality of two different blankets, it is recommended that you decide on a test form to print—whether it is the
current job you are running on the press or an actual standard print
quality test form.
The test should contain color bars and any additional print quality
targets that will provide relevant information. Start the press using
your current blanket(s) installed to correct specifications (use preset
6
Offset Printing Blanket Overview — continued
torque wrenches), collect acceptable printed sheets and then stabilize
the press.
Once this has been accomplished, stop the press. Change only the
printing blankets. Make no other adjustments on the press. Restart the
press and after making ‘fine tuning’ adjustments, collect acceptable
printed sheets.
Compare:
• solid ink densities
• dot gain values
• print contrast values
• solid ink traps
• dot structures
• quality of solids
• any additional measurements
deemed necessary
Make note of any major press adjustments that had to be made during
the test. If acceptable results are achieved, it’s recommended you
continue running the blanket on different printed forms to collect more
data for further evaluation.
• Release—blanket release characteristics are not easily measured. But
when a blanket has poor release properties, it is easily detected. Poor
blanket release can cause problems such as:
• dot distortion
• poor registration
• blanket piling
• hickeys
• streaks or lines in the print and paper
• substrate distortion
There are two ways to improve the release properties of a printing
blanket. One way is by chemical means. The other is by mechanical
means.
Chemical release characteristics are determined by the chemical
composition of the rubber surface. The surface of the blanket and
construction of the carcass determine mechanical release properties.
Chemical and mechanical release properties can often compliment
each other.
For a printing blanket manufacturer, it can be difficult to produce
blankets with both excellent print quality and release characteristics
because the two properties sometimes work against one another.
Example: a blanket with a smooth surface may transfer a very sharp
dot to the substrate; but its mechanical release properties may suffer
because of the greater surface contact between the blanket and the
substrate. Therefore, the blanket would require excellent chemical
release properties to compensate for the deficiency in the mechanical
release properties.
7
When evaluating release characteristics of a blanket, you should check
for the following problems:
• Excessive web flutter that can’t be corrected with web tension
• Uneven marks or lines in the print
• Excessive sheet curl on a sheetfed press
• Poor registration between units
• Excessive paper and/or ink buildup on the blanket surface
• Poor print quality (dot distortion, smoothness of solid, loss of
detail)
• Durability and Longevity—seldom do printing blankets stay on the
press long enough to wear out. Blankets are usually changed because
they have been damaged in some way and no longer produce acceptable print quality.
A printing blanket can be damaged in a number of ways.
Problems such as a smash caused by a wrinkled sheet or web break,
changes in web or sheet size, foreign material passing through the
press, poor blanket maintenance and improper packing or tensioning
procedures are just a few examples.
A blanket can be repacked with new or additional under-packing and
not have to be changed. Although there are many temporary “repair”
products and methods to swell or rejuvenate a damaged blanket, these
repair methods are only temporary at best. When a blanket is smashed
or damaged, eventually it will have to be changed.
Press downtime incurred in trying to repair a blanket and the possibility of inferior print quality caused by a damaged blanket can sometimes far exceed the cost of a new blanket.
It’s recommended you keep a printing blanket record or logbook at
each press to track printing blanket usage. The record should include
the following information:
• style and blanket manufacturer
• installation and removal date
• blanket gauge
• packing thickness
• press unit position
• total number of impressions
• reason(s) for removal
You may want to include additional information. A blanket record or
logbook will be valuable in troubleshooting problems, evaluating
blanket performance and providing detailed information on blanket
consumption.
(Day International can provide charts or help you develop a tracking
system for your pressroom.)
8
2
Blankets
On The Press
Proper installation and packing of the offset
printing blanket is the foundation for consistent
print quality and blanket performance. Here are
guidelines to remember:
Plate Cylinder
Packing...
Plate.......
Bearer
Proper Gauges—having the proper gauges to measure blanket and
under-packing is essential. Properly calibrated and well-maintained
gauges will quickly help you determine blanket-related print problems.
Cylinder
Undercut
Plate over Bearer
Blanket
Cylinder
Impression
Cylinder
9
Bearer
Blanket......
Packing. ......
Bearer
Blanket
over
Bearer
Cylinder
Undercut
Packing—one of the most misunderstood, yet critical
elements, is packing. When under-packing, you should know
three things:
• blanket thickness
• required packing thickness
• required print measures
Packing height is important because the ideal packing height
delivers optimal print pressures and enhances the following
properties:
• print quality
• resistance to piling
• blanket life
• smash resistance
• release
• plate wear
• tension control and feed
• registration
Improper packing can have a negative effect on any or all of
the above. And, there are certain printing problems that can
be caused or aggravated by improper packing, such as:
•
•
•
•
•
•
•
Weak print
Pinholes
Piling
Poor trap
Emulsification (mottling)
Slurring
Dot gain
•
•
•
•
•
•
•
Poor Register
Pressure wrinkles
Sheet/web release
Web control (tension)
Web breaks
Excessive plate wear
Plate cracking
• Undercut and Iron-to-Iron Settings—the undercut and iron-to-iron
settings are two of the most important press specifications on the
printing press. All packing and print pressures are set in reference to
these two specifications. Ideally, these specifications should be posted
on or near the press, so that decisions can be made quickly and
accurately.
Improper blanket packing is a common problem that leads to poor
print or press performance. There are no packing guidelines that apply
one hundred percent of the time. However, knowing the blanket gauge
and packing thickness and print/squeeze pressures will alleviate most
pressure-related problems.
Again, packing heights are dependent upon the following:
• Type of press
• Bearer or iron-to-iron settings
• Basis weight; type of paper being used
• Type and brand of blanket being used
• Tensioning and torquing procedures
Steps to proper packing:
1.
2.
3.
4.
5.
NOTE: To achieve
optimum print
quality, bearer
pressures must
be set after the
blankets and plates
are packed to the
desired heights.
Know the undercut of both the plate and blanket cylinders. If you do
not know this dimension, check the press manual or measure the
cylinder with a packing gauge.
Measure the thickness/gauge of the blanket and the under-packing,
as well as the plate and under-packing. Don’t assume the product
information stamped on the back of the blanket is correct. Measuring
takes very little time in relation to the problems that may occur during
a press run.
Allow extra packing to compensate for blanket stretch and “run in.”
All blankets will stretch, losing gauge anywhere from 0.001"-0.002".
Use a packing gauge to properly check blanket height to help you to
determine the proper squeeze pressure.
Adjust the packing to best accommodate your printing conditions
and requirements. To achieve the best results from your press, it’s
crucial for you to learn the variables and their associated measurements.
• Blanket Tensioning—this procedure is critical to attaining top quality
print and consistent performance. Proper torquing, along with proper
packing procedures, will provide quality results in print values, registration, tension control, sheet release and overall blanket life. Torquing
gives you consistency not only from unit-to-unit, but uniformity around
the whole cylinder.
10
Blankets On The Press — continued
Example:
To Calculate Packing Height of Blanket
Desired blanket height above bearer:
Blanket cylinder undercut:
Subtotal
English
.004
.081
.085
Metric
0.10
0.06
2.16
Blanket thickness (subtract):
.077
.008
1.96
0.20
.002
.010
.077
.087
0.05
0.25
1.96
2.21
Subtotal
Add .001 to .002 for draw down:
Packing Thickness Needed:
Blanket Thickness:
CalculatedTotal Packing Height:
Plate:
Blanket:
Total
.001
.004
.005
above bearer
above bearer
total plate-to-blanket squeeze
Blanket:
Blanket:
Total
.004
.004
.008
above bearer
above bearer
total blanket-to-blanket pressure
plus paper caliper
Blanket Tensioning
Measuring the blanket on the press is accomplished several ways using
several different types of gauges. The number one priority should be to
have a gauge that provides accurate and repeatable measurements.
Gauges should be handled carefully and kept clean, calibrated and in a
secure case or cabinet when not in use. Remember that the blanket is the
heart of the press—every adjustment that is made on a press is reflected
in the blanket’s performance.
2.
1.
When measuring blanket thickness on the press, always wash the blanket
first. Ink buildup, paper dust and coating on the blanket can affect the
accuracy of the readings. These critical measurements are in thousandths
of an inch, so the reading has to be exact.
There are three critical areas on the blanket that must be measured while
the blanket is on press. These areas will determine if the blanket is
installed properly and whether the correct height has been obtained to
achieve the correct print pressure. The three areas are:
1. lead edge of the blanket at the gap
2. trail edge of the blanket at the gap
3.
11
3. half gap or 180
o
from the gap
If the three measurements are within .0015" in these three areas, the
blanket is circular. If these measurements have a greater range, then the
blanket is egg shaped (due to over-tightening).
When blankets are improperly tensioned, several problems may occur.
Poor register, feed, release, susceptibility to smashes and on-press
damage may result. All of these problems shorten blanket life and affect
performance.
When installing blankets on presses with double reel rod lockups, be
sure to insert the same length of blanket in both ends. Tighten both sides
at the same time to insure the blanket is stable or square on the cylinder.
On presses that only have trail end reel rod lockups, it is easy to overtighten one end and create a “step” from the lead edge to the trail edge.
This can cause register problems, “gear streaking” or “cylinder bounce”
on the printed sheet.
No-Pack Blankets
It’s important to be consistent when tensioning or torquing no-pack blankets. When printing blankets are over-tightened, the gauge is lost quickly.
With no-pack blankets, there is no option of “packing up.” Extreme care
should be exercised when installing no-pack blankets. A quick check with
a packing gauge will ensure proper installation and avoid subsequent
printing problems.
Torquing No-Pack Blankets
Torquing is recommended in all applications—using a torque wrench
when installing the blanket allows consistency and avoids problems
related to improper tightening. Following are the recommended torquing
procedures:
Step I
Install the blanket as usual. Use a torque wrench set to
recommended specifications
Step II
With the press set “impression on”, run the blanket around
3 to 6 times; leave press set to “impression on” and
retighten the blanket. Now the blanket should be very snug
Step III
Set press to “impression off” and re-torque the blanket
Step IV
During the first press stop, re-torque the blanket
Step V
The blanket is now torqued—further tightening should not
be needed unless a printing problem is evident
12
Blankets On The Press — continued
Tips on Torquing
• Make sure torque wrench is calibrated to proper setting
• Keep blanket lockups in good working condition
• On presses equipped with double-reel rod lockups, torque both sides
of the cylinder at the same time
• Check blanket height after installation
Procedure for Lengthening
or Shortening Print
The rule is:
• To lengthen print—take packing from plate cylinder
and put it under blanket
• To shorten print—transfer packing from blanket cylinder
to the plate cylinder
• Always maintain the same squeeze pressure
Troubleshooting Print Problems
When troubleshooting to identify the source of printing problems, some
specifics should be checked. Following are “vital signs” to check:
• Verify undercut or iron-to-iron settings and bearer settings
• Verify blanket gauge and packing thickness. Don’t assume the gauges
indicated on the blankets are correct - measure them!
• Verify the blanket height on the problem cylinder and measure it in the
three critical areas (lead edge gap, trail edge gap and half gap). The
blanket should be inside of .0015" in all three areas
• Review blanket installation procedures
• Review torquing and tightening procedure
• Check bench packing of blanket or what gauge blanket and packing
thickness are being used to obtain desired blanket height on press after
tensioning
• Check gauge of new blanket on press
The above steps should identify problems with the printing blanket or the
packing. If all these areas appear in good order, check the other press unit
systems for possible causes. The most common blanket problems are:
• Over-tightening and improper tensioning of blankets
• Improper packing procedures
13
• Poor blanket maintenance
• Major mechanical (blanket lockups - bearer/iron-to-iron settings)
• Incorrect blanket specifications (wrong sizes/bars)
Remember…
proper
procedures
provide good
results and
save time and
money!
Establishing Procedures
Always maintain specific procedures to achieve consistency. Procedures
are put into place to eliminate costly mistakes and provide a baseline to
correct problems through certified change.
The printing blanket is the final component in achieving quality printing.
Understanding the principles of the blanket and its role in the printing
process will ensure high quality, performance and consistency.
14
3
Proper care
of the offset
printing
blanket can
be a deciding
factor in its
successful
application
and use.
Care and Maintenance
of Offset Blankets
Storage and Handling Do’s And Don’ts
• Upon receipt of new printing blankets, check to ensure that the blankets are
the correct style, length, width and thickness. Make sure they have been
punched and barred as required.
• Whenever possible, store printing blankets in the original shipping tube or
carton. These containers will protect the blankets from being exposed to
light and from the abuse that can occur in normal storage and handling.
• Don’t force more than the original quantity of blankets into the shipping
tube or carton. This may cause stress or wrinkles in the printing surface—
the type of damage that can eventually show up on the printed sheet.
• If you must store the printing blankets flat, out of their shipping tube or
carton, lay the blankets face-to-face and fabric-to-fabric. Cover the blankets
to shield them from dirt and light.
• When storing printing blankets out of the container, do not remove the tape
from the bar ends of the blankets. The tape is applied to protect the printing
surface from possible scratches.
• When handling the blankets, do not create sharp bends or folds in them.
This may cause surface defects that can also show up on the printed sheet.
• Store printing blankets in a cool, dry, clean area. This will protect them and
ensure that they are usable when needed.
On Press Do’s and Don’ts
• Follow all safety rules and precautions when installing the blanket.
• Make sure the blanket and blanket cylinder are free from any foreign
material.
• Tension the blanket properly with a calibrated torque wrench or torque
limiter. Over-tensioning may result in premature blanket failure and undertensioning may result in print problems.
• After installation, hand-wash the surface of the blanket with warm water to
remove any powders or dust that remains on the surface of the blanket.
This will allow for a much faster and trouble-free start-up.
• Clean the blankets as often as practical. This will help eliminate possible
ink build up at the gap or outside the printing area.
• Don’t use ink knives or other sharp tools in cleaning the blanket. These
items can damage the blanket surface.
• Use as little blanket wash as possible. Excess wash can soak into the back
of some blankets causing them to swell.
• If using automatic blanket washers, make sure they are set properly at all
times. Incorrect settings can damage the surface of the blanket.
15
• Use only solvents or washes that are formulated specifically for blanket
cleaning. Harsh solvents may cause face swelling and they may actually
soften the rubber on the blanket face, making it sticky. This will contribute
to poor paper release.
4
Measures to Ensure Optimal
Blanket Performance
• There are many reasons why you should be conscientious in the
selection, use and care of offset printing blankets. Since the blanket is
the final point of contact between the press and the printed substrate, it
has a direct effect on your printing quality.
• Blankets should be viewed as an investment. Inferior printing blankets
and poor usage techniques can cost you money in downtime and can
eventually cost you valuable customers.
• Since printing blankets directly influence dot reproduction, proper
selection, careful mounting and packing can make a tremendous
difference in the quality of your printed job.
• A printing blanket’s condition, as a result of use, affects printing. Low
spots, glaze or dry ink buildup (caused by poor washing procedures),
tackiness (caused by the use of harsh washes), embossing or
debossing (caused by incompatibility of pressroom chemicals) may all
affect your reproduction quality.
• Improper packing can also seriously affect printing quality: a difference
of as little as .0015" may result in a noticeable change of color values.
It’s important you check the gauge of the blanket and select the proper
under-packing sheet to obtain the correct on-press blanket and packing
height. Improper packing of the plate can also cause serious print
quality problems. Always use a packing gauge to ensure proper printing pressures.
• Poor mounting or installation procedures of the blanket can cause
problems. It’s important to use a torque wrench when mounting
blankets to prevent over or under-tightening that can cause excessive
gauge loss or loose blankets on the press.
• Be sure the blanket cylinder is free of any debris or foreign materials
that can cause high spots. If a blanket is loose on the cylinder, not only
does the loose fit affect register and dot reproduction, but it also
allows water and solvent penetration into the fabric backing of the
blanket. This can cause swelling, uneven printing pressures and
eventual blanket delamination.
Check with the blanket manufacturer or supplier for proper
blanket torquing recommendations.
• With proper use and care, your blankets should produce hundreds of
thousands of quality impressions. Pressroom conditions, practices and
type of printing projects all affect blanket life.
• Types of presses definitely have an effect on blanket life. Only a careful
and accurate count of impressions noted faithfully on a data chart
will indicate true blanket life. Variables such as type of printed form,
inks, papers, smashes and unusual conditions will have an effect on
16
Roller and Blanket Maintenance — continued
blankets and should be noted. Solvents in washes should also be
carefully considered.
• There are many important factors to be aware of when selecting a
printing blanket. Factors like uniform thickness, surface texture,
squareness, stretch characteristics, resilience, ink receptivity, release,
chemical resistance and compatibility with other pressroom supplies
should all be carefully considered when selecting the proper printing
blanket for your application.
• Properties such as thickness and squareness can be checked before
use, but it’s not always easy to determine whether a problem is the
result of improper care or poor blanket quality. Continued checking
will determine the facts.
• Proper handling and care of printing blankets is good common sense.
It’s recommended that blankets are stored in the tube or carton in
which they are shipped—the tubes are designed to protect the blankets
from damage.
• Store the blankets in a dry, cool location of your facility—it’s important
to keep blankets away from moisture sources such as steam pipes, air
conditioners or heaters. Moisture will affect the fabric and cause poor
blanket performance.
• Avoid exposing the blankets’ surface to direct sunlight or UV rays.
Such radiation can cause blanket damage. If you prefer to store blankets on a shelf, be sure to lay them rubber side to rubber side and
fabric side to fabric side.
• Opinions vary on how and when to wash blankets. It’s impossible to
establish hard and fast rules. Depending on the type of stock and ink
used, a blanket may have to be washed more or less often. Obviously, a
blanket should always be washed when it fails to transfer ink properly.
As a general guide, a blanket should be washed:
• Before it’s used for the first time
• When print quality deteriorates or buildup is noticed on the surface
• When, for any other reason, the press stops for a length of time
• Before changing printed forms
A soft rag, or any soft, absorbent material which does not shed lint when
soaked with blanket wash, is essential:
• Don’t use too much solvent
• Start washing in the center of the blanket and work out to the edges
• Avoid excess wash on the edges of the blanket
• Remove excess wash from the blanket’s surface with a clean, dry rag
Using proper care and maintenance techniques can extend printing
blanket life. Remember, the printing blanket plays an important role in the
quality of your work and the success of your pressroom.
17
5
Varn Product
Overview
The single most important step in ensuring continuous, high-quality
output in the pressroom is a focused press maintenance program. Varn
fountain solutions, washes and specialty products are formulated and
field-tested for virtually every printing application, whether a large heatset
web press, a mid-sized sheetfed or a small offset duplicator.
The following sections detail and describe each of these products and
their recommended use in the pressroom.
18
6
Roller and Blanket
Maintenance
Printing on a poorly maintained press is more than just a dirty job for
most printers. Glazed rollers and blankets can cause major problems,
such as improper ink transfer, which can lead to lower output and increased overall costs in wasted paper, ink, hourly wages, overhead and
irreplaceable press time.
It doesn’t matter how much you spend on pre-press electronics in an
attempt to achieve the best image possible, if your rollers cannot transfer
ink properly, the printed image will suffer. Keeping rollers and blankets
clean helps minimize or eliminate these concerns. The following tips can
help define maintenance parameters and assist in improving an existing
maintenance program.
Solvents for Washing the Press
There are four basic solvent types for washing up the press. In most
cases, the same wash can be utilized for both rollers and blankets.
1.
2.
3.
4.
19
Water-miscible washes – the most popular type in the industry,
water-miscible (or mixable) washes can generally be blended with 20%50% water. Featuring moderate drying speeds, these washes remove
ink and water-soluble glazes, such as gum and paper fillers in one
wash-up. Water-miscible washes are available in detergent, low-VOC,
vegetable oil and non-photo-chemically reactive formulas.
Straight solvent washes – an economical alternative to watermiscible washes. They cannot be mixed with water and are, therefore,
used directly from the container. This type of wash features fast drying
and good penetration of dried ink.
Two-step washes – applied in a sequence, these washes are used for
deep cleaning and color changes. The two-step process has been
designed to cut wash up time in half when changing from dark to light
colors in the same unit. Step one is a blend of detergents, solvents and
water in a heavy-viscosity wash. Step one solvents quickly “bite” into
the ink and remain on the rollers, while the detergents penetrate into
the roller surface and carry the ink and glaze residue down to the
wash-up blade for removal. The drying time for step one solvents is
slow. Step two is a water-miscible rinse wash that is used to completely remove all of the step one wash and to prepare the roller
surface for the next color.
Environmental washes – were designed to meet increased demands
created by recent environmental legislation. They use a broad base of
ingredients, most of which are designed to lower VOCs, raise flash
points, and reduce environmental and health concerns in the pressroom.
The Ink Roller Train
The ink roller train consists of an ink fountain with ink keys, an ink
distribution blade and roller, a ductor roller and alternating hard and soft
distribution rollers. Proper ink distribution begins at the ink fountain. If
the blade is nicked, the roller surface scored or pitted, or the fountain
ends clogged, the ink distribution will be inconsistent and uncontrollable. The ink keys must be routinely cleaned to keep these parts moving
smoothly.
The metered ink from the ink fountain passes between the alternating
hard and soft rollers. This ink film is milled, split and delivered all the
way to the form rollers, then to the printing plate. If the press is to print
with the expected quality, the soft rubber rollers must maintain their
correct shape and durometer (or softness).
• Proper roller durometer is critical to the performance of the ink roller
train. Hard rollers require more pressure to attain the proper “stripe” or
contact with the next roller surface.
• When more roller pressure is required to get the proper “stripe,”
friction and heat will build up and further damage the roller’s ability to
split and deliver the ink film.
• Hard rollers transfer less ink than soft rollers, and ink feed must be
increased to achieve proper density. Increased ink feed results in
increased water feed.
• Changes in the shape of a roller cause the “stripe” to become uneven
and the ink will not transfer evenly across the press. This shows up on
the printed sheet as an inability to achieve proper ink density from side
to side.
There are some
mechanical
problems that
can contribute to
ink control
difficulties such
as the existing
condition of the
rollers, roller
settings, bearings
and the correct
lubrication of the
press. However,
the biggest
culprit is glaze.
• Rollers change shape as they age. Swelling on the ends of the roller
usually indicates that the center of the roller has collapsed due to the
leaching of plasticisers. Plasticisers are an ingredient in the rubber
compound that gives the roller its softness and shape. Exposing rollers
to harsh solvents and deglazers depletes the amount of plasticisers in
the rubber and the rollers can harden or collapse.
Glaze
Glaze is a condition when the roller surface becomes contaminated with
dried chemical and solid debris. This condition hardens rollers prematurely and creates a surface that will not carry and transfer ink properly.
Glaze can also create a surface on the roller that attracts water and
displaces ink, otherwise known as “roller stripping” (the inability to carry
ink in a particular area). There can also be color variation problems
20
Roller and Blanket Maintenance — continued
caused by ink density changes during the run without any changes in the
ink and water balance settings. Glaze will also contribute to color contamination problems when old ink debris discolors the ink in use.
Causes of Glaze
The elements that contribute to glazing have evolved over the years.
Today’s printers are using alcohol substitutes, new ink formulas, low
VOC products and alkaline papers. All these factors can increase glaze
problems. Today, glaze can be divided into three categories: solvent
soluble, water soluble and acid soluble.
• Solvent soluble glaze – contains mostly oil-based ingredients,
including ink particle pigments, oils, resins, varnishes, binders, dryers
and some oil-soluble ingredients from fountain solution of alcohol
replacement components. Solvent washes will dissolve these byproducts.
• Water soluble glaze – contains gum arabic, fountain solution debris,
dried alcohol replacements, spray powders, paper coatings, paper lint,
some minerals, and some surfactants. The regular use of water in the
wash up process, along with regular use of a detergent-type deglazer,
will remove this type of glaze.
• Acid soluble glaze – is most often caused by calcium from inks or
calcium carbonate, an ingredient in alkaline papers. Calcium combines
with carbon dioxide and forms calcium carbonate. Calcium carbonate
is not dissolved by water or solvents, and must be removed with a
product specifically designed to eliminate calcium. When not removed,
the build up of calcium carbonate will contribute to roller durometer
increase, roller stripping, plate blinding and color variation.
Recommended Roller and Blanket
Maintenance Procedure
Roller:
• Perform regular wash up during a press run as needed
• Perform a color change wash-up when printing radical changes in
color (such as dark to light), to prepare for varnish or to remove
metallic pigments
• Perform a maintenance wash-up twice weekly to ensure high
productivity from the press
Blanket:
• Perform regular wash up during a press run as needed
• Perform a maintenance wash-up twice weekly
21
7
The following
information serves as
a printer’s guide to
understanding the
U.S. EPA Control
Technique Guidelines
(CTG) and the
Alternative Control
Techniques (ACT) as
used to measure
volatile organic
compounds (VOC) in
the pressroom.
VOC and
Vapor Pressure
VOC
A VOC is a Volatile Organic Compound, also known as VOM - Volatile
Organic Material, or VOS - Volatile Organic Substance.
Volatile Organic Compounds are solvents derived from petroleum or
other sources, which evaporate into the atmosphere. When exposed to
sunlight, VOC can react with emissions from vehicles to cause air pollution problems. VOC emissions are measured in pounds per gallon, grams
per liter or percentage by weight. The test method approved for VOC
measurement of pressroom chemicals in the United States is EPA Test
Method 24.
Test Method 24 is used to measure the VOC in a liquid and
proceeds as follows:
• One-half gram (a few drops) of the liquid is weighed into an open cup
and placed in an oven heated to 110ºC (230ºF) for one hour.
• The cup of liquid is removed, cooled and weighed again. Whatever has
evaporated by weight is considered to be volatile and that weight is
called the VOC content.
• If the liquid to be measured contains water or chlorinated solvents,
additional tests and measurements are performed to establish VOC
content.
• Water is not a VOC and chlorinated solvents are exempted.
Control Technique Guidelines
Control Technique Guidelines (CTG) are a series of industry standards
that the U.S. EPA would require each state to adopt to help lower VOC
emissions. The first draft CTG standard proposed for the lithographic
industry suggested that all blanket and roller washes contain no more
than 30% VOC. Most press washes, including water miscible washes and
the popular quick-drying solvent washes, contain close to 100% VOC.
Forcing all printers to use washes containing less than 30% VOC was a
drastic change.
The technology to produce washes that cleaned effectively with less
than 30% VOC content did not completely exist at that time. This became
evident when the U.S. EPA sought comment from printing industry
organizations like GATF, PIA and manufacturers like Varn. The EPA
learned that 30% VOC washes were ineffective, reduced productivity
and actually created pressroom safety hazards by their inherent slippery
and greasy nature.
As a result of their findings, the EPA published a supplementary document to be used in conjunction with the original draft CTG when determining methods for reducing VOC emissions in the pressroom. This
supplementary document is the Alternative Control Technique (ACT).
22
VOC and Vapor Pressure — continued
The ACT guidelines, published in September of 1994, permit individual
states to use vapor pressure measurements of press washes as an
alternative to reduce VOC emissions. Vapor pressure measures the
volatility of a product or its ability to evaporate. As an example of the
effect of vapor pressure, consider a lawn mower’s gas container and how
it expands in hot weather. When opened, the fumes vent from its cap.
Volatile Organic Compounds also have a tendency to expand and try to
evaporate. As temperatures rise, vapor pressure also rises.
Measuring Vapor Pressure
A glass measuring device, similar to a barometer, is used to measure the
vapor pressure of a Volatile Organic Compound. The evaporation of a VOC
in this closed device pushes a thin column of mercury up a glass tube.
The vapor pressure is measured as the height of the mercury in millimeters. The EPA vapor pressure rule states that any blanket or roller wash
which measures less than 10 mm of mercury (Hg) at 20°C (about room
temperature) can be considered a low VOC product, equivalent to a
product with less than 30% VOC content as measured by Test Method 24.
Fountain Solutions, Alcohol
and Alcohol Substitutes
Both the Control Technique Guidelines (CTG) and the Alternative Control
Techniques (ACT) cover VOC measurement in fountain solutions, isopropyl alcohol and alcohol substitutes. The vapor pressure alternative does
not apply to fountain solutions.
How This Affects Printers
In the U.S., each state is free to use either the CTG or ACT guidelines or a
combination of both. Since these choices are subject to change by state
legislative action, individuals are encouraged to contact the environmental enforcement group in their state (these groups may be listed under
various names, e.g.; Department of Natural Resources, State’s Environmental Protection Agency, Department of Air & Water Quality, etc.). It is
recommended to send a letter asking for adoption of the vapor pressure
rules on blanket and roller washes, which gives the greatest flexibility in
selecting products while helping to reduce air pollution. Use of a 30% VOC
wash will be required if your state elects to remain with Test Method 24.
Limits for alcohol and alcohol substitutes in fountain solutions are only
slightly different between CTG and ACT versions. The bottom line for
printers is that alcohol must either be eliminated or severely limited,
depending on which guidelines are used. If more than the prescribed
amount of alcohol is still being used, it is strongly suggested, in the
interest of economics, to begin eliminating its use at this time.
23
8
Fountain Solution
Chemistry
The Function of Fountain Solution
in Offset Printing
Water Hardness
Scale
(in parts per million of
calcium carbonate)
Soft
0-100
Medium 101-200
Hard
201 and greater
The dampening of the offset plate is one of the most critical factors in the
production of quality offset printing. Through the use of a fountain
concentrate mixed with water, a solution is created that performs several
essential functions on press. The most important of these is the wetting
and desensitizing of the non-image area of the plate. The fountain solution also lubricates the plate and blanket. This reduces image wear and
prolongs plate life. Fountain solution cools the plate, helps to maintain
the working properties of the ink and aids in proper blanket release,
which in turn, reduces piling on the blanket.
Water
Approximately 98% of a working fountain solution is water. Tap water can
range from very hard to soft. Water quality can vary seasonally, monthly,
weekly and even daily. It is critical to be aware of water conditions at all
times to insure quality, trouble-free printing. Ideally, printers should use
treated water for purity and consistency.
Measuring Fountain Solution Concentration
Establishing the proper pH in the dampening fountain is essential to plate
performance and print quality. Maintaining the correct level of acid in the
solution is absolutely necessary to permit the gums in the fountain
solution to bond to the non-image surface of the plate.
The pH scale is logarithmic to the 10th power and each whole number
represents a ten-fold increase or decrease in acid or alkaline concentration.
Buffers
Buffers are ingredients that Varn adds to fountain concentrates to help
stabilize the pH level of the mixed solution. These buffers reduce the
tendency of pH to drift to either the acid or alkaline range due to the
introduction of contaminants found in paper coatings, inks and additives.
Buffering is also useful when using one-part alcohol reducing fountain
solution concentrates. The stabilized pH level allows for the addition or
reduction of concentrate within the working range without concern for pH
level. Careful attention should still be paid to mixing concentration. The use
of buffers makes it necessary to use conductivity as your measurement.
Conductivity
This is the measurement of the ability of a fountain solution to conduct
electricity. Checking fountain solution conductivity is particularly impor-
24
Fountain Solution Chemistry — continued
tant when running a buffered fountain solution that tends to hold a
constant pH reading despite great changes in water quality. Conductivity
will indicate even the slightest variation in fountain water quality.
There is no universal conductivity standard for all fountain solutions.
Individuals must develop a standard for their plant using their water
source.
The graph illustrates that pH remains constant from 3.5 ounces per gallon
up to 8 ounces per gallon. Thus, monitoring only pH level through the
press run would be useless in determining actual fountain concentration
or contamination level. Conductivity measurement, however, shows a
linear progression of concentration or contamination level.
Alcohol
Alcohol has been identified as a Volatile Organic Compound. As a VOC,
alcohol pollutes our pressrooms and our environment. Federal, state and
local environmental regulations have taken effect and by now, most
printers have tried or are currently using alcohol replacements.
Alcohol Replacement
In order to replace alcohol, there are a few important considerations.
Alcohol is a single raw material that, by itself, performs several functions
for the pressman. When running with alcohol replacements, the pressman must perform some functions. To be successful, a roller and blanket
maintenance program must be implemented (see Chapter 6, Roller and
Blanket Maintenance). Reset rollers to the light side of the specification
and invest in new rollers if needed. Keep recirculators clean on a regular
basis. Invest or maintain chillers and recirculators. Be ready to re-balance
the press and turn down the ink feed.
25
9
Spray
Powders
The primary function of anti set-off powders is to form a space between
sheets as they leave the press. This separation traps air between the
sheets to speed ink drying. It also breaks the suction effect of static and
prevents the wet ink from setting off onto the sheet above. The ability of
the powder to perform this function is a result of a combination of
particle size, density and the number and nature of particles applied.
Usually, as the weight of the printing stock increases, the recommended
particle size also increases. The largest users of spray powder are folding
carton printers and large sheetfed offset printers.
Spray Powder Elements
Varn Anti Set-off Powders are blends of pure food starches with anticaking and flow agents added. Varn Coated Powders go through an
additional manufacturing step that microencapsulates the individual
particle to change its performance.
How the Powers are Different
Varn manufactures two types of anti set-off powder:
• The “R” series - regular or uncoated powders
• The “C” series - coated powders
Both are available in a wide range of particle sizes. The coated, or treated
powders, are quite different in their working properties from the regular
powders, but both types perform well in electrostatic (electronic) units as
well as conventional air-pulsating guns.
Varn coated powders are microencapsulated which completely changes
the working properties of the powder. They repel water and are attracted
to the surface of the ink, giving better ink set-off protection with increased mileage. Coated powders flow and perform better in electronic
units than conventional powders and, because they resist humidity, they
won’t clog air guns. For a given micron size, coated powders offer a
smoother finish and are ideal for the last pass-through on large, multiunit presses.
Micron/micron Range
A micron is a measurement equal to one millionth of a meter (1/25,400 of
an inch). Varn powders are listed in micron ranges because most powders contain quantities of smaller and larger particles due to of the nature
of the raw materials used. Varn controls particle size by careful selection
of the proper raw materials and by a unique screening operation during
manufacturing.
26
Spray Powders — continued
When to Use Coated and Regular Powders
Coated powders cover the majority of applications. They are ideal for use
by large corrugated printers, folding carton manufacturers, most fourcolor work printed on four-color presses and most large sheet-fed printers running heavy lifts of paper. The grade of coated powder is determined by the weight of paper, the print coverage and the press spray
equipment. As a rule, the grade of coated powder, and therefore the
micron size, increases as the weight of the paper, print coverage or the
height of the delivery pile increases. Coated powders should not be used
when sheets are laminated or have certain coatings. Check with the
coating manufacturer on whether to use coated or uncoated powders.
Varn’s regular powders are used in work-and-turn printing, multiple-color
printing on presses with fewer units than the number of colors being
printed, or any printing that requires multiple passes through the press.
There are some exceptions, but trapping, especially on small presses,
improves with regular grade spray powder.
Troubleshooting
There are a variety of pressroom conditions that cause or affect ink setoff. Among them are:
• Room Temperature – the higher the room temperature, the faster the
ink will dry. Temperatures above 70°F (20°C) speed ink drying.
• Room Humidity – relative humidity above 60% retards ink drying. A
difference of 10% in humidity can appreciably alter drying time.
• Stock Humidity – this condition greatly affects ink set-off. A job that
sets off in the center of the sheet and not on the outside edges is often
caused by humidity variations in the stock. Stock that does not lie flat
is more susceptible to ink set-off.
• Static Electricity - static accumulation on the sheet can be a prime
cause of ink set-off.
• Stock Weight – folding carton stock and other heavy stock has a
greater tendency towards ink set-off.
• Ink Drying - ink formulation, drier type and content are prime factors
in reducing ink set-off.
27
TROUBLE
CAUSE
CURE
Overall ink set-off
Not enough powder
applied or too fine a
micron size
Increase amount of powder and/or increase
micron size
Too much ink applied
or incorrect ink/water
ratio
Cut back ink, check fountain pH
Add Compound 747 (ink conditioner)
Pile is too high
Set out job in smaller lifts
Spray pattern is
uneven
Check for blocked nozzles
Powder does not
cover the full sheet
Check timing of powder unit
The powder splashes
or lumps
Foreign matter in powder or spray unit out of
adjustment
Powder builds up on
the blanket or spray
nozzles
Too much powder
applied or incorrect
micron size
Cut powder volume or increase micron size
Powder clogs in
applicator spray bar
or on nozzles
Powder is damp
Clean dispensing unit and refill
Poorly graded powder
Change to Varn powders for quality and
performance
Ink set-off only in
specific areas of the
sheet
Air supply is
inconsistent
Clean unit and air lines or have unit serviced
Powder interferes
with overprinting,
laminating or is
blocking the plate
Particle size too large
or wrong grade used
Refer to application chart and select uncoated
(regular) grade and size. Regular grade
powders will perform better in overprinting,
varnishing or laminating
Rough finish on
printed sheet
Micron size is too
large
Decrease micron size
Powder flying or
dusting
Excessive amount of
powder or too much
Reduce powder volume or micron size and/or
reduce air supply
28